1. Field of the Invention
The present invention relates to a multiplexing transmitter having a cross connecting function. More particularly, it relates to a multiplexing transmitter having functions for dropping or demultiplexing an optional time slot of a channel of a high-ordered group to optional channels of a low-ordered group or adding and multiplexing optional time slots of a low-ordered group to a channel of the high-ordered group in the reversed direction (hereinafter it is referred as to an add/drop cross connect function), and further, for turning optional time slots of the low-ordered group to optional channels of the low-ordered group (hereinafter, it is referred to as a hair pin cross connecting function).
2. Description of the prior Art
While a multiplexing transmitter having a large-capacity transmitting function has been developed, it is required to realize higher channel protection with a cheap investment in recent years.
Therefore, there has been a strong demand for a system having a higher channel protection power which employs an optical multiplexing transmitter according to a channel protecting method operating in a ring configuration so called as a PATH-SW method, in which the number of optical fibers can be reduced to pare down costs of a system. In here, the channel protection power is defined as a ratio of channels by which a communication can be continued even when a failure occurs such as disconnection of a fiber transmission line.
According to the expansion of an add/drop cross connect function, demands for modifying a specific path channel or installing more specific path channels in networks originally constructed as different optical communication networks, respectively, after starting operation in order to cope with development of the communication markets in an appropriate area and efficient use of installed, residual optical fiber transmission paths.
For example, there is a case where more path channels are installed between transmission terminals respectively connected to each optical multiplexing transmitter including a cross connect function located on each node of two separate ring-formed networks, each of which is originally formed as an independent network.
In this case, a path channel of a high-ordered group should be maintained in bi-directions of upward and downward on West and East sides in each ring-formed network. Therefore, it is easy to maintain a path channel of the high-ordered group even if a failure occurs on a transmission path connecting to either West or East side in ring-formed network.
However, a transmission path connecting between two separate ring-formed networks is not formed as a ring, and has no redundancy. Because of that, if a failure occurs on the transmission path connecting between the two separate ring-formed networks, it becomes impossible to maintain a path channel between the two separate ring-formed networks.
As mentioned above, when more path channels bestriding over the two different ring-formed networks are installed, it is easy to protect the failed path channel even if a failure occurs on either upward or downward transmission path in each ring-formed network.
However, when a failure occurs on a transmission path bestriding over the two separate ring-formed networks, it is not easy to protect the failed channel. In this way, there are one case wherein it is easy to protect the failed channel, and the other case wherein it is not easy.
Therefore, it is required to give a protection even to a transmission failure on path channels in the above-described form. Simultaneously, it is also required to monitor a failure to distinguish whether the failure should be immediately protected or not in order to reduce a maintenance cost. Additionally, it is required to have a failure warning function with an information of a priority for protecting failed channels, and the priority is determined according to degree of influence on a service to end users.